The present invention relates to a device for decoding signals received at the output of a transmission channel after duobinary encoding of a sequence of binary digital signals. This duobinary signal decoder is particularly suitable for use in television signal receivers in accordance with the so-called D2-MAC/packet standard.
In television, the introduction of colour was at that time realised by adding what are referred to as colour difference signals or chrominance signals to the luminance signal Y. But an examination of the spectrum of the video signal thus obtained has shown that the luminance and chrominance information components may mix resulting in interferences which are known as cross-luminance and cross-chrominance.
The recently developed MAC transmission standard obviates this drawback. This new standard which is intended to ensure the broadcasting of a set of information components, both analog (luminance, chrominance) and digital (sound, synchronizing signals, any commentaries or subtitle elements, . . . ) within the band limits of a transmission channel, provides a time-division multiplexing of these different information components in a television line, for example in the following manner:
a digital information component: 10 microseconds; PA1 chrominance: 17.5 microseconds; PA1 luminance: 35 microseconds. PA1 (a) first, second and third memories for storing respective conditional probabilities P(y/0), P(y/1), P(y/2), receiving at the instant kT, wherein k is the order of consecutive digital samples, a duobinary signal y when a duobinary signal x=0, 1 or 2 has been transmitted upstream of the transmission channel, said storage memories being addressed in parallel by the received signal y for deriving the corresponding probabilities. PA1 (b) first and second selection and storage circuits for two conditional probabilities, which at the instant (k+1)T correspond to the two most probable binary sequences--or paths--, the first circuit being provided at the output of the first and second storage memories and the second at the output of the second and third storage memories; PA1 (c) a circuit for progressively reconstructing the binary paths thus selected and selecting the most probable path of the two; PA1 (b.sub.1) first and second multipliers respectively third and fourth multipliers which at their first inputs receive the output from the probability storage memories and calculate the quantities P(y/0).multidot.LI(0,k) and P(y/1).multidot.LI(1,k) or P(y/1).multidot.LI(0,k) and P(y/2).multidot.LI(1,k) respectively, in which quantities LI(0,k) and LI(1,k) designate probabilities which are associated with said two paths which were previously selected by these same selection and storage circuits and applied to second inputs of said multipliers; PA1 (b.sub.2) first and second comparators, respectively for comparing the output values of the first and second respectively the third and fourth multipliers, for the purpose of selecting the larger of the two probabilities present at the output of the first and the second multipliers and of the third and fourth multiplier respectively; PA1 (b.sub.3) first and second storage registers, respectively, for said associated probabilities thus selected, whose outputs are fedback to the second inputs of the multipliers; PA1 (c.sub.1) two shift registers for the respective storage of the two binary sequences corresponding to the two probabilities selected by the selection and the storing circuit, respectively; PA1 (c.sub.2) a third comparator for comparing the output signals of the two probability storage registers which are respectively selected by the selection and storage circuits, for the purpose of selecting the larger of these two probabilities and the one of the two binary sequences being stored in said shift register which corresponds thereto.
There are several versions of this MAC standard. France has adopted as broadcast standard, the so-called D2-MAC/packet version, in which the bit stream is of the order of 10 mbit/s and the coding used is duobinary. Unfortunately, the performance of this D2-MAC/packet standard in difficult receiving conditions (for example satellite transmissions having a low signal-to-noise ratio) is restricted by the audio selection of the transmission system in which said standard can be used. Tests have indeed demonstrated that for a signal/noise ratio of approximately 13 dB on an amplitude-modulation link the decoding of the digital signals in the receiving section of the transmission system is effected with an error rate of 10.sup.-3, which is considered to be the receiving limit for sound, while the picture still remains acceptable.